Spinal cord stimulation (SCS) is a well-accepted clinical method for reducing pain in certain populations of patients. Spinal cord stimulator and other implantable tissue stimulator systems come in two general types: radio-frequency (RF)-controlled and fully implanted.
The type commonly referred to as an “RF” system includes an external RF transmitter inductively coupled via an electromagnetic link to an implanted receiver-stimulator connected to one or more leads with one or more electrodes for stimulating tissue. The power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, is contained in the RF transmitter—a hand-held sized device typically worn on the patient's belt or carried in a pocket. Data/power signals are transcutaneously coupled from a cable-connected transmission coil connected to the RF transmitter and placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation.
In contrast, the fully implanted type of stimulating system contains the control circuitry, as well as a power supply, e.g., a battery, all within an implantable pulse generator (IPG), so that once programmed and turned on, the IPG can operate independently of external hardware. The IPG is turned on and off and programmed to generate the desired stimulation pulses from an external portable programming device using transcutaneous electromagnetic or RF links.
In both the RF-controlled or fully implanted systems, the electrode leads are implanted in the epidural space, or alternatively near the dura of the spinal cord. Individual wires within one or more electrode leads connect with each electrode on the lead. The electrode leads exit the spinal column and, when necessary, attach to one or more electrode lead extensions. The electrode leads or extensions are typically tunneled within the subcutaneous tissue along the torso of the patient to a subcutaneous pocket where the receiver-stimulator or IPG is implanted. The RF transmitter or IPG can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue, and in particular, the dorsal column fibers and dorsal root fibers within the spinal cord. The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient.
Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied in SCS include electrical pulse parameters, which may define the pulse amplitude (measured in milliamps or volts depending on whether constant current or constant voltage is supplied to the electrodes), pulse duration (measured in microseconds), pulse rate (measured in pulses per second), pulse shape, and burst rate (measured as the stimulation on duration per unit time). Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some SCS systems, and in particular, SCS systems with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the receiver-stimulator or IPG, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current (or voltage) in different relative percentages of positive and negative current (or voltage) to create different electrode configuration, and in particular, fractionalized electrode configurations.
As briefly discussed above, an external control device, such as an RF controller or portable programming device, can be used to instruct the receiver-stimulator or IPG to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the external device, itself, can be adjusted by manipulating controls on the external device itself to modify the electrical stimulation provided by the SCS system to the patient. However, the number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician or patient.
To facilitate such selection, the clinician generally programs the external control device, and if applicable the IPG, through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the receiver-stimulator or IPG to allow the optimum stimulation parameters to be determined based on patient feedback and to subsequently program the RF transmitter or portable programming device with the optimum stimulation parameters. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
For example, in order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the RF transmitter or IPG to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the IPG, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the stimulation region or areas correlating to the pain. Such programming ability is particularly advantageous after implantation should the leads gradually or unexpectedly move, thereby relocating the paresthesia away from the pain site. By reprogramming the external control device, the stimulation region can often be moved back to the effective pain site without having to reoperate on the patient in order to reposition the lead and its electrode array.
Even when using a computerized programming system, programming or reprogramming the external control device or IPG may be a difficult task. Oftentimes, a clinician may identify a stimulation parameter set where a patient is obtaining great paresthesia, but when the clinician subsequently returns to this stimulation parameter set, even within the same programming session, the patient may no longer receive the same paresthesia. In some cases, the patient may not feel any paresthesia at all when the clinician returns to this stimulation parameter set.
Candidate reasons for the change in paresthesia over time are neurologic phenomena, such as accommodation, adaptation, and habituation, which entail a diminished neural response over time when there exists continuous input (in this case, electrical stimulation) due to cellular and synaptic mechanisms. For the purposes of this specification, we will use the term “accommodation” to generally refer to any mechanism that diminishes neural response due to continuous input. Currently-used methods to avoid accommodation include a 1-hour rest interval to avoid accommodation of nerve fibers (see Benedetti, Fabrizio M D, et al; Control of Postoperative Pain by Transcutaneous Electrical Nerve Stimulation After Thoracic Operations. Ann Thorac Surg 1997; 63: 773-776). However, this is an unrealistic solution as it would more than double the time needed for a programming session. Increased programming time leads to higher workloads for the clinicians and increased costs. Furthermore, once neurological accommodation has occurred, there are currently no techniques to reverse or otherwise manage the accommodation.
There, thus, remains a need for an improved method and system that avoids, reverses, or otherwise manages neurological accommodation during the programming of neurostimulation devices.